The present invention relates to a power steering apparatus comprising a motor as a power-source of the steering assisting force.
Power steering apparatus is to alleviate the labor burden required for steering by rotating a steering wheel and to receive favorable steering feeling such apparatus is equipped in many automobiles. Furthermore, in recent years there has been practically employed an electric power steering apparatus utilizing an electric motor which is driven according to the operation of the steering wheel, instead of the hydraulic actuator extensively used conventionally.
Such power steering apparatus has a merit of elastically meeting to the change of the properties of the assisting force according to the operating conditions such as HI/LOW of the vehicle speed, LARGE/SMALL of the steering angle, and the like by control of motor driving for steering assistance, however, in realization of such control, a transmission system for transmitting the rotation of the motor for steering assistance to the steering mechanism is essential.
The motor for steering assistance is provided in linkage with the steering wheel. The motor are connected with the middle portion of a steering column which rotates according to steering of the steering wheel or provided at the middle portion of the steering shaft, both ends of which is connected with traveling wheels (in general, right and left front traveling wheels), for steering by shifting in the axial direction (e.g., a rack shaft in the rack-pinion type steering mechanism). In the former, however the rotation of the motor for steering assisting is to be transmitted to the steering column which is a rotary member, and while there is a benefit that the above transmission system can be simply constituted by a general gearwheel reduction mechanism, the portion of the steering column outside the saloon is small, so that it is difficult to secure a installation position for the transmission system including said motor, and there is an inconvenience that the application range could be limited.
Against the above, however the latter has complicated constitution of the transmission system for converting the rotation of the motor for steering assistance into the shift of the steering shaft in the axial direction, it has large freedom for arranging positions of the transmission system including the motor over the axial length of the steering shaft arranged to the right and left in the bow of the vehicle, and has an advantage of applicability to extensive kinds of vehicles. However, even in this constitution, curtailment of the space for arranging the transmission system including the motor as above is an important task. For example, as disclosed in Japanese Patent Laid-open Publication No. S61-191468, a power steering apparatus aiming at curtailing the installation space by using a ball screw mechanism for the above transmission system has so far been proposed.
This power steering apparatus employs a portion of the steering shaft as a male screw member on the outer periphery of which a thread of the ball screw is formed, and on the outside of the male screw member, there are arranged the nut members which are screw connected through a large number of balls with restraint given to the movement in the axial direction in the housing for supporting the steering shaft, and the rotation force from the motor for steering assisting is transmitted to the nut member, so that according to the rotation the steering shaft is shifted to the axial direction by utilizing the screw advance of the ball screw. By this, it is possible to constitute the transmission mechanism that gives the large reduction ratio inside said housing compact, and by mounting the miniaturized motor near the housing of the steering shaft, the requirements for reducing the layout space as described above can be met.
However, in the conventional constitution as above, there is required high precision for forming the ball screw thread in the outer periphery of the steering shaft, for which a large number of steps are required for processing; moreover, great amount of labor is required for adjusting screw engagements between the ball screw threaded male screw member and the nut member.
Though the balls which are filled in the ball screw thread in the nut member move in rotation in the screw groove according to the screw advance of the balls screw to smooth the movement of the steering shaft, but at the same time the balls deflect by their rotational movement in the direction of movement of the steering shaft. In order to prevent this deflection, there are provided bypass mechanisms for moving the balls in circulation at the right and left ends in the nut member, thereby supporting smooth movement in rotation of the balls. However, the bypass mechanisms constituted in the nut member are extremely complicated, involving the problems that due to a large number of portions constituting the said mechanisms increases are caused in the number of steps for assembly and cost.
Furthermore, there is another problem that in the above bypass mechanism and ball screw thread, because there is no retaining member to keep the distance between the balls constant, the balls collide against one another and the noise generated thereby is large.
The present invention has been made in reflection of the situation as above. An object of the invention is to provide a power steering apparatus which generates low noise and which can securely convert the rotation of the motor for steering assistance to the movement of the steering shaft in the axial direction by the transmission system having a simple constitution to take the place of the ball screw mechanism, thereby it is possible to meet the requirements for curtailing the installation space of the transmission system including the steering assistance motor and simplifying the adjustment work at the assembly.
A power steering apparatus of the present invention has a constitution to assist steering by transmitting rotational force of a motor driven according to steering to a steering shaft supported in a housing with its rotary movement restrained and by moving said steering shaft in the axial direction, comprising a rotary cylinder supported in said housing with its movement in the axial direction restrained and rotated coaxially with said steering shaft by the rotational force transmitted from the motor, a plurality of feed rings fit and held in said rotary cylinder, made eccentric in different directions with respect to the axial center of the rotary cylinder, being rotatable on the axis that is parallel with said axial center, a spiral groove provided on the outer circumference of the steering shaft to be engaged at one position in a circumferential direction with a projection provided around the inner surface of the feed ring, and adjusting means for adjusting the eccentricity amount of the plural feed rings by the operation from the outside of the rotary cylinder.
FIG. 1 shows an explanatory view for illustrating the principle of converting the rotation of the feed ring of the present invention to the movement in axial direction of the steering shaft. The steering shaft 101 is supported under allowance of shift in the axial direction only. The feed ring 163 has a steering shaft 101 inserted through its inner circumferential portion, the inner diameter is larger than the outer diameter of the steering shaft 101, and the feed ring 163 is disposed eccentrically to the axial center of the steering shaft 101 so as to come into contact with the outer circumferential surface of the steering shaft 101 at one place of the inner circumferential surface of the feed ring 163. Further, the axial center of the feed ring 163 is provided with inclination by an angle xcex8 to the axial center of the steering shaft 101.
When the feed ring 163 having such constitution rotates around the axial center of the steering shaft 101; for example, in the white base arrow mark direction around the axial center of the steering shaft 10, the feed ring 163 is to rotate around the steering shaft 101 while being in contact with the steering shaft 101 always at the same position on the inner circumferential surface thereof. As the feed ring 163 shows a tendency to make screw advance in the direction inclined by the angle xcex8 in itself to the direction of rotation like a female screw, and the friction force F acts to the inclination direction at the contact portion, so that the shift of the steering shaft 101 in the axial direction can be attained by the component force Fsin xcex8 in the axial direction of the steering shaft 101 at that time.
Such a shift can be attained in the same manner by setting the axial center of the feed ring in parallel with the axial center of the steering shaft, and engaging the feed ring with the spiral engaging groove provided on the outer circumferential surface of the steering shaft at an angle with the axial direction. The arrangement of the feed ring having the axial center parallel with the steering shaft can be realized with high positional precision inside the rotary cylinder which rotates coaxially with the steering shaft, and further, the feed ring can be favorably engaged with the engaging groove on the outer circumference of the steering shaft. Accordingly, it becomes possible to convert the rotation of the motor for assisting the steering transmitted to the rotary cylinder to the movement in the axial direction of the steering shaft with high efficiency.
Another power steering apparatus of the present invention is characterized by having three or more feed rings, and the two feed rings held on both sides of the rotary cylinder in the axial direction are inserted in fitting holes in the axial direction through openings aligned on both end faces of the rotary cylinder and held therein, the fitting holes being formed with circular sections corresponding to the respective outer shapes of the feed rings and on both ends of the rotary cylinder, and other feed rings are inserted in the fitting parts in a radial direction through the openings aligned on the outer circumferential surface of the rotary cylinder and held therein, the fitting parts being formed with semi-circular bottom faces corresponding to the respective outer configurations of the feed rings and in the intermediate part of the rotary cylinder.
In this invention, by the insertion in a fitting hole or a fitting part formed on the rotary cylinder, three or more feed rings can be held with correct positional relations kept with the axial center of the rotary cylinder and favorably engaged in the engaging groove on the outer circumference of the steering shaft, and the rotation of the motor for steering assistance can be converted to the movement in the axial direction of the steering shaft with high efficiency.
Another power steering apparatus of the present invention is characterized in that the feed rings are antifriction bearings comprising a plurality of rolling members interposed between the inner ring and the outer ring, and further in that a part or the whole of the antifriction bearings have the outer rings formed in one-piece with the rotary cylinder.
In this invention, an antifriction bearing for universal use having good shape precision is used as a feed ring to have movement conversion effected with high efficiency. Further, the outer ring of the antifriction bearing is integrated with the rotary cylinder keeping the correct positional relationship with the axial center of the rotary cylinder to have it securely engaged with the engaging groove on the outer circumference of the steering shaft.
Another power steering apparatus of the present invention is characterized in that the rotary cylinder is supported in the housing by the four-point contact ball bearing formed in one-piece utilizing a part of the rotary cylinder as an inner ring.
In this invention, a rotary cylinder holding a plurality of feed rings is supported in a housing by the four-point contact ball bearing with the rotary cylinder itself as an inner ring, thereby favorably securing the coaxial property with the steering shaft and securely engaging the feed ring held by the rotary cylinder with the steering shaft.
Another power steering apparatus of the present invention is characterized in that the motor is equipped with a cylindrical rotor coaxially connected with the rotary cylinder on one side, or that the motor is disposed outside the housing, and is constituted to transmit its force to the rotary cylinder through gearwheel means.